1. Field of the Invention
The present invention relates to a technique capable of, when a drive motor is locked as a result of the rotation of a mirror rotation unit being mechanically locked in a power-folding mirror, correctly detecting this locked state.
2. Description of the Related Art
A power-folding mirror constitutes an outer mirror (mainly a door mirror) for a vehicle, and is constructed so that a mirror rotation unit (mirror housing and a part that rotates together with the mirror housing) is rotationally moved from a return position (use position or extended position) to a folded position, and also is rotationally moved from the folded position to the return position by means of driving a motor. The rotation of a power-folding mirror usually starts as a result of a motor being started by a driver's switch operation in the vehicle. Upon the start of the motor, the mirror rotation unit automatically moves from the return position toward the folded position (or from the folded position toward the return position). When this movement is mechanically locked by a stopper at the end position of the movement (the folded position or the return position), the locking is detected and the driving of the motor is automatically stopped.
For a method for detecting locking of a motor for a power-folding mirror, a method utilizing a value of current flowing in the motor (for example, the method described in Japanese Patent Laid-Open No. 8-40146) is commonly used. In other word, the value of current flowing in a motor during power being supplied to the motor becomes greater (i.e., lock current flows) when the motor is locked, compared to when the motor is not locked. Utilizing this principle, the current flowing in the motor is converted into a voltage via a resistance provided in the power supply line and monitored, and when the voltage value crosses a predetermined threshold value set to a voltage between the voltage when the motor is locked and the voltage when the motor is not locked, the motor is determined as being locked and then the power supply to the motor is halted.
Capabilities required for a power-folding mirror include ensuring the operation under every combination of a temperature in a predetermined temperature range (for example, −30° C. to +80° C.) and a drive voltage in a predetermined drive voltage range (for example, 8 V to 15 V). However, current flowing in the motor greatly fluctuates within this operation-ensured range whether the motor is rotating or locked, and it also greatly changes depending on the motor type, and accordingly, it is difficult to set an optimum threshold value for determining whether or not the motor is locked. In particular, when the difference between the current during the motor being operated and the current during the motor being locked is small, the threshold value setting is further difficult. Therefore, in order to avoid a phenomenon in which the mirror rotation unit stops in the middle of rotation because of an erroneous determination regarding whether or not the motor is locked, the setting may be configured so that power will continuously be supplied to the motor for a while even when the motor has been determined as being locked. However, in this method, even when a motor is actually got locked, the motor is continuously supplied with power for a while, so lock current will continuously flow for a long period of time, causing damage of the motor. Also, in a method in which whether or not the motor is locked is determined based on lock current, the threshold value must be set for every design change involving a change of motor, and thus, threshold value setting has been troublesome.
The present invention has been made in view of the aforementioned points and is intended to provide a lock detection method and a lock detection circuit for a motor for a power-folding mirror, which are capable of correctly detecting a motor-locked state.